The present invention relates to a method and a device for detecting and analyzing deposits in liquid-bearing systems.
Industrial plants, like power plants, steel mills, pulp making plants or paper making plants, usually comprise means for conducting or storing fluids, e.g. pipe lines or fluid containers. It is a known issue that organic and inorganic matter deposits on the inner walls of these means for conducting or storing fluids, whereby an accumulation of deposits, like e.g. fouling or scaling, at least partially blocks the flow through the conducting means and conducted or stored fluids may become contaminated. This is an unwanted occurrence that causes a number of operational problems such as plugging of equipment, inefficient usage of chemicals, increased utility costs, lost production due to downtime, corrosion, and downgraded products from increased dirt counts.
In principle but not limited to, one can distinguish between fouling deposits on the one hand and scaling deposits on the other hand. Fouling deposits are organic deposits which often occur in the form of biofilms in aqueous systems. Such biofilms substantially consist of micro-organisms, e.g. bacteria, algae, fungi and protozoa. Contrary thereto, scale depositions occur from inorganic matter that have been identified include e.g. complexes of calcium (carbonate, oxalate, sulfate, silicates), aluminum (silicates, hydroxides, phosphates), barium sulfate, radioactive radium sulfate, and silicates of magnesium.
In order to avoid the accumulation of fouling deposits and in particular the growth of biofilms, biocides are added into the fluid concerned. Scaling deposits can be removed by adding chemical deposit control agents based on homopolymers, copolymers and terpolymers of acrylic acid, methacrylic acid, maleic acid and aspartic acid. Furthermore the chemical deposit control agents can be based on organic phosphonates and their derivatives, as well as on polyphosphates.
The dosage of these biocides and chemical deposit control agents has to be accomplished very carefully and conservative because they are very expensive and pose a health hazard. It is thus necessary to distinguish between various types of deposits and to determine the thickness of respective deposits.
A method and a device for high precision measurement of a characteristic of a fouling or scaling deposit inside a fluid vessel is disclosed in the prior art document WO 2009/141 135 A1. An ultrasonic emission signal is emitted by an ultrasonic transducer towards a reflecting area inside the fluid vessel and a distance between the ultrasonic transducer and the reflecting area or between the ultrasonic transducer and a deposit onto the reflecting area is measured by means of evaluating the time-domain reflective signal of the reflecting area or of the deposit covering the reflecting area. The measured distance is compared to a reference distance which has been measured in an initial calibration measurement step without any deposits onto the reflecting area. The difference between the measured distance and the reference distance is a measure for the thickness of the deposition. A disadvantage of this method is that the real distance between the ultrasonic transducer and the reflective area changes e.g. with the temperature or the pressure inside the fluid vessel. Therefore, the current distance between the ultrasonic transducer and the reflective area at the time of measurement cannot accurately defined by a previously measured reference distance. Consequently, the measurement of the thickness of the deposits comprises an unknown offset depending on operational conditions, like pressure and temperature.
In order to distinguish between scale and fouling deposits the prior art document WO 2009/141 135 A1 further discloses a device comprising two different ultrasonic transducers, wherein each of the ultrasonic transducers cooperates with a separate reflecting area. The two reflecting areas are provided with different acoustic impedances. As scaling and fouling deposits have also different acoustic impedances, a comparison between the amplitude of the time-domain reflective signal measured by the one ultrasonic transducer and the amplitude of the time-domain reflective signal measured by the other ultrasonic transducer enables to distinguish between fouling and scaling deposits. A disadvantage of said device and method is that the device has to be provided with two different measuring sections which leads to higher equipment costs, increased installation space and further errors sources, because the operational conditions are not equal everywhere in the liquid-bearing system and rather depends on the exact position along a fluid pipe, for instance.